Modularize and restructure Users and Roles and Security chapters of Broker configuration guide

The Users and Roles and Security chapters of Broker configuration guide have content relating to setting up authentication and authorization for on-premise broker deployments. However, the content across the two chapters is very fragmented and not organized based on those workflows.

A revised outline is shown in the Broker modularization plan here: https://docs.google.com/document/d/1XepmkrdQ9tfhLcvTGYjXJNgyt9k03PF7dPQMmeqdZjY/edit#heading=h.q99xtswpdkx0

An initial implementation plan for the modularization work is shown here: https://docs.google.com/spreadsheets/d/1kifex0wPro_zlvG7UOoVW1UF-uZlRi_rqX9BYJnaW88/edit#gid=1775581729

This issue is to track the work of:

• Modularizing the existing content
• Restructuring the content based on the outline in the modularization plan
• Some high-level (i.e., non-technical) cleanup of existing content to better meet style guidelines